Processes for the selective absorption of H.sub.2 S from gaseous mixtures utilizing alkaline liquid absorbent containing amino compounds are known.
It is also known to use a liquid absorbe containing a severely hindered amino compound for the selective removal of hydrogen sulfide from normally gaseous mixtures. See, for example, U.S. Pat. No. 4,405,585 in which the severely hindered amine is a secondary ether alcohol such as the ones claimed in U.S. Pat. No. 4,471,138, and U.S. Pat. No. 4,405,583 in which the severely hindered amino compound is a di-secondary aminoether, the teachings of all of these patents are hereby incorporated by reference.
Although the alkaline absorbents containing the amino compounds are capable of removing acidic gases such as hydrogen sulfide from hydrogen sulfide-containing gaseous mixture, it is progressively more difficult particularly at low pressures to remove hydrogen sulfide at normal operating conditions to a level such that the absorbent-treated gaseous mixture (i.e., exit gas) contains less than about 10 volume parts per million (vppm) hydrogen sulfide. When it is desired to produce a gas having less then 10 vppm hydrogen sulfide, the treated gas, for example, a Claus tail gas, containing more than 10 vppm hydrogen sulfide is typically incinerated to convert the remaining hydrogen sulfide to SO.sub.2. Therefore, it would be advantageous to improve the efficiency of the known alkaline amine absorbents to increase the amount of hydrogen sulfide that they are capable of removing at normal operating conditions so as to yield a treated gas having less than about 10 vppm, preferably less than 1 vppm, hydrogen sulfide.
J. H. Dibble's European Patent Application 84107586.4 (Publication No. 0134948) published March 27, 1985 discloses that the absorption of hydrogen sulfide at low pressures by certain alkaline absorbents, which may contain an alkanolamine, is enhanced by using in the absorbent an acid or an acid-forming material having a pKa of 6 or less in an amount sufficient to protonate less than 22% of the alkaline material to produce a treated gas having less than 10 vppm hydrogen sulfide.
U.S. Pat. No. 4,618,481 issued Oct. 21, 1986 to Exxon Research and Engineering Company discloses the absorption of hydrogen sulfide by the use of an alkaline absorbent composition comprising a severely hindered amine and an amine salt to produce a treated gas having less than 10 vppm hydrogen sulfide.
U.S. Pat. No. 4,153,674 discloses the addition of strong acidic compounds such as acids and ammonium salts thereof to aqueous alkanolamine absorbent solutions, see column 6, lines 33 to 48.
U.S. Pat. No. 2,722,500 discloses removing acid gases from hydrocarbon gases by using an alkanolamine salt of a polybasic acid having a high ionization constant, for example, phosphoric acid, and hydrochloric acid. It discloses that it is convenient to react the acid in advance with the amine.
U.S. Pat. No. 3,139,324 discloses an absorbent solution for H.sub.2 S comprising an ethanolamine and a polybasic acid such as phosphoric acid. The ethanolamine is present in an amount between 0.20 gram mole and 0.02 gram mole per liter.
U.S. Pat. No. 3,848,057 discloses an absorbent solution comprising ethanolamine and a basic salt. The acid gases may contain H.sub.2 S and SO.sub.2.
U.S. Pat. No. 4,080,423 discloses a process for absorbing acidic gases using a basic component and a weakly dissociated compound such as a weak acid or a salt thereof. Acids and salts listed as suitable include phosphoric acid, and sulfurous acid. As shown in Example 1, the basic component may be ethanolamine and the salt can be sodium phosphate, and the acid gas to be purified may comprise H.sub.2 S and SO.sub.2.
It now has been found that a treated gas having less than 10 vppm H.sub.2 S can be obtained from a low pressure gas and that in treating fluids comprising H.sub.2 S as well as CO.sub.2, an increasing H.sub.2 S selectivity can be achieved by utilizing an absorbent composition comprising an alkaline severely hindered amine mixture in combination with its salts with a strong acid or in combination with a severely hindered aminoacid.